Hydrorefining catalysts have been hitherto produced such that an active metal, which has, for example, a demetallation ability and a hydrogenation ability, is impregnated on a carrier composed of porous inorganic oxide such as alumina. The hydrorefining is performed such that hydrocarbon oil is brought into contact with the hydrorefining catalyst in the presence of hydrogen, in which it is possible to remove hetero elements contained in the hydrocarbon oil, i.e., sulfur, nitrogen, and metal components (for example, vanadium, nickel, and iron). Various investigations have been made for such a catalyst concerning, for example, the active metal, the property of the carrier, the pore structure, and the method for impregnating the active metal in order to improve the ability to remove the hetero elements.
As for the method for impregnating the metal, Japanese Patent Application Laid-Open No. 9-500815 discloses a method in which a carrier is impregnated with an impregnation solution containing cobalt acetate and phosphomolybdic acid to perform vacuum drying at 160° C. In this method, the catalyst is not calcinated. Japanese Patent Application Laid-Open No. 6-31176 discloses a catalyst composition having a hydrotreating activity and containing a catalyst which does not substantially contain free water and which includes salt and/or complex of metal belonging to the VIII group and heteropolyacid of metal belonging to the VI group supported on a support substance of inorganic oxide. It is described that citrate may be used as conjugate base for acid which constitutes the salt of metal belonging to the VIII group. However, in this method, the catalyst composition is not calcinated.
WO97/47385 (Japanese Patent Application Laid-Open No. 2000-511820) discloses a method for impregnating a carrier with an impregnation solution dissolved with a solid obtained by evaporating and drying a solution containing phosphomolybdic acid, cobalt carbonate, and citric acid, followed by being calcined at 400° C. in a nitrogen atmosphere. In this method, it is necessary to firstly prepare an aqueous solution of a compound represented by a general formula MxAB12O40 (M is cobalt and/or nickel, A is selected from phosphorus, silicon, and boron, B is molybdenum and/or tungsten, and x is not less than 2 when A is phosphorus). Subsequently, the aqueous solution is treated with a reducing agent. More specifically, crystalline phosphomolybdic acid H3PMo12O40.13H2O is dissolved in water, to which a COCO3 solution, citric acid as a buffer, and metal cobalt as a reducing agent are added to prepare a dark blue solution. After that, this solution is evaporated and dried to recover a bulk solid corresponding to Co7/2PMo12O40.xH2O. Subsequently, this solid is dissolved in water to obtain the impregnation solution thereby (Example 1). On the contrary, in the present invention, it is unnecessary to prepare the specified compound represented by the general formula described above when the impregnation solution is prepared, and it is unnecessary to perform the treatment with the reducing agent.